Many industrial plants have rotating shafts which are subjected to temperature differences between two regions, causing large thermal stress variations on these shafts.
This is especially the case for the primary coolant pumps of a nuclear power station which convey hot water at high temperature.
In their top part, these primary coolant pumps include a heat exchanger, called a thermal barrier, which cools the water feeding a hydrodynamic bearing and the rotary seals with the longitudinal shaft. There is therefore a transition region between the hot water and the cold water located at the bottom of the thermal barrier.
That part of the shaft located in this transition region is consequently subjected to a large thermal gradient which promotes thermal instabilities that may create cracks in the shaft.
To reduce this risk of cracking, a thermal protection shield is placed over the shaft in the region where the thermal gradient is greatest.
Hitherto, this thermal protection shield was formed by a ring of stainless steel surrounding the shaft in said transition region. This solution does not suffice for completely safeguarding against the risk of cracking, since after a few years of operation cracks may appear at various places in the shaft below this ring.